Space-division multiplex full-duplex local area network

ABSTRACT

An optical wireless local area network according to the present invention is an optical wireless local area network for interconnecting a plurality of terminals having a line-of-sight optical communication function. In the optical wireless local area network, a base station including an angle diversity light reception function and a plurality of optical transmitters each having directionality is provided. The intensity of each of the plurality of optical transmitters can be separately modulated.

TECHNICAL FIELD

The present invention relates to a space-division full-duplex local areanetwork for interconnecting information terminals having adirected/line-of-sight optical communication function for use in anoffice or home environment.

BACKGROUND ART

At present, optical wireless communication, which uses infrared ray indata transmission between information terminals in offices or homes, inconformity with the Infrared Data Association (IrDA) standard iswidespread. In such optical wireless communication, an opticaltransmitter-receiver includes a light emitting diode (LED) having acertain directionality as a transmitter and a photodiode (PD) having anappropriate field of view as a receiver.

Two terminals each including such an optical transmitter-receiver areplaced a short distance from one another, facing each other. Theterminals perform line-of-sight communication by intensity modulationwith direct detection (IM/DD). Such directed/line-of-eight opticalcommunication is most advantageous to a portable terminal which requireslow power consumption, small size, low weight, and low cost, andtherefore is widely used. To date, the communication rate of thedirected/line-of-sight optical communication is 4 Mbps, and thetransmission range thereof is 1 m. In the future, directed/line-of-sightoptical communication will be developed to achieve a communication rateof 100 Mbps and a transmission range of 5 m. Directed/line-of-sightoptical communication is increasingly widespread among end users throughmore and more various applications handling moving pictures and thelike.

A LAN (local area network) in which communication is performed by IM/DDusing infrared light as a medium has been vigorously developedthroughout the world.

FIG. 7 shows various forms of Infrared communication, and corresponds toFIG. 1 in Publication 1 (Joseph M. Kahn et al., Proceedings of the IEEE,pp. 265–298, 1997). FIG. 7 is divided into upper and lower rows(line-of-sight and non-line-of-eight, respectively) depending on whetheror not line-of-sight communication is used. FIG. 7 is also divided intocolumns (directed, hybrid, and non-directed) depending on whether atransmitter-receiver has directionality. In an optical wireless LAN inwhich a plurality of terminals are wirelessly connected to each accesspoint, light needs to be avoided from being blocked by a barrier orpeople walking in a network space, for example. Therefore, as shown inthe lower right corner of FIG. 7, light is diffused and transmitted in awide field range, and the light is received by a receiver having a widefield of view. Communication in the form of anon-directed/non-line-of-sight diffuse link is promising. Alternatively,a hybrid system shown in the middle of FIG. 7 is used in which atransmitter uses a directional beam and a receiver has a wide field ofview. These systems have merit in the construction of flexible LANs, butrequire high-cost transmitter-receivers having a high level ofpower-consumption, or multi-stage transponders. These systems findacceptance in heavily used indoor environments such as offices,hospitals, or schools.

Such existing LAN systems employ their own communication forms andcommunication protocols which are not compatible with the IrDA standardswhich are widely used for portable terminals and the like. Even thoughIrDA terminal users desire to interconnect a plurality of terminals,their IrDA communication functions cannot be used. A whole system mustbe newly introduced. Recently, Kahn et al. proposed in Publication 1that a simultaneous link is achieved using space division multiplexingamong a plurality of terminals having a directed/line-of-sightcommunication form shown in the upper left corner of FIG. 7. In thisproposal, data transmission among all of the terminals is mediated by anangle-diversity receiver and multi-beam transmitter, which togetherconstitute a so-called optical wireless hub.

FIG. 8 shows two examples of an angle-diversity receiver which is amajor component of an optical wireless hub, and corresponds to FIG. 22of Publication 1. In either example shown in FIG. 8, any angle at whichsignal light comes from corresponds to the coordinate of the position ofone of a plurality of photodetectors.

Of the examples shown in FIG. 8, an example in which an imaging lenshaving a relatively high spatial resolution is used will be particularlydescribed, with reference to FIG. 8( b), which is a diagram showing aconfiguration of an imaging receiver and FIG. 8( d), which is a diagramschematically showing the spatial resolution of the imaging receiver ofFIG. 8( b). In this case, the imaging lens is designed so that anoptical signal from any direction is converged to a signal focusingplane. Therefore, an optical signal incident to the imaging lens at acertain angle is detected by a certain cell (and/or cells in thevicinity of the cell) of a monolithic photodetector array which outputsa signal in response to the incident optical signal. The detected signalis amplified by a preamplifier array subsequent to each cell. Of suchdetected signals, a signal having the highest intensity is selectivelyprocessed so that signal sources having different angles with respect tothe imaging receiver can be separately identified. In principle, anN-to-N simultaneous communication is possible.

However, there are various problems to be overcome in order that theportable terminals are directly incorporated into a high-speed LANhaving a random multiple access capability. One of the problems is thattransmission and reception cannot be simultaneously conducted in thecommunication between the portable terminals in conformity with the IrDAstandard, limiting the communication to a half-duplex communication. Themajor physical factor of such a problem is that transceivers must besimple, small, and inexpensive and therefore the transceivers cannothave a structure for preventing transmitted light from diffracting andreturning to the transceivers that have transmitted the light (e.g., areceiver and a transmitter are positioned at a sufficient distance fromeach other).

Further, in conventional optical wireless LANs, optical transmission andreception may be conducted using a signal optical channel (e.g., diffuselight in a single wavelength band covers an entire network area). Suchcommunication is limited to one-way 1-to-N (broadcast) communication.Time division multiplexing (TDM) is introduced to the communication,thereby making it possible to conduct time division multiplex access(TDMA). When a system interconnects a plurality of terminals, it isdifficult to significantly increase a transmission rate between eachterminal and the power consumption of the whole system is increased. Thesystem may interconnect a plurality of terminals using a so-calledcellular communication system in which a network space is divided intospace cells using a plurality of beams having a certain level ofdirectionality. In this case, when TDMA is conducted in half-duplexcommunication, it must be verified that other terminals have alreadyconducted a communication, just before each terminal startscommunicating on the LAN. Such a verification procedure is calledcollision avoidance. Even when the collision avoidance procedure isconducted, if a terminal in a bad communication state (hidden terminal)exists within an area, a communication error may occur.

Even when a code is assigned to each communication channel (CDMA) or acarrier frequency is assigned to each communication channel (FDMA),i.e., multiplexing using an electric circuit, a communication capacityper user is limited. In this case, signal processing is very complex,and the power consumption of the whole system is inevitably increased.Even when CDMA or FDMA is combined with the cellular communicationsystem in the LAN, simultaneous communication among a plurality ofterminals causes interference among the signals. Therefore, theconventionally well-known collision detection procedure isindispensable. Therefore, a waiting time and an extra signal processingare required for each terminal, so that it is difficult to provide asatisfactory high-speed LAN environment.

However, in wavelength division multiple access (WDMA) in which acommunication wavelength is assigned to each channel, multiple accesscan, in principle, be simultaneously conducted in a diffuse link. Inthis case, the wavelength of a light source of each transmitter needs tobe variable. Conversely, when a light source of each transmitter has aconstant wavelength and a plurality of wavelength bands are used, areceiver requires a bandpass filter in which only single wavelengths areselected from all of the wavelength bands used in a link and a centerwavelength of transmission is variable. Such functions are not easilyachieved in a single device at low cost. Accordingly, a transmitterincluding a plurality of light sources each having a constant wavelengthand a receiver including a plurality of filters each having constantbandpass characteristics are required for each terminal, so that apractical system is not achieved.

The object of the present invention is to provide a high-speed andlarge-capacity LAN in which a plurality of terminals can besimultaneously interconnected without producing significant load on theterminals and in which improved communication abilities (longer distanceor higher speed) of the terminals are directly achieved, using themerits of the directed/line-of-sight optical communication which iswidespread for use in portable terminals.

DISCLOSURE OF THE INVENTION

According to the present invention, in a transmitter-receiver unit ofeach terminal, a receiver includes an optical filter for cutting signallight from its own transmitter. Space-division communication among allof the terminals is conducted using an optical wireless hub having aspace division capability. The wavelength band of each light source of amulti-beam transmitter of the optical wireless hub contains a spectrumcomponent different from the spectrum components of all of thewavelength bands used by each terminal. Thereby, a simultaneous andfull-duplex multi-access LAN among a plurality of terminals performingdirected/line-of-sight communication is achieved. Also, the speed ofone-to-one directed communication between terminals is increased as muchas possible.

According to one aspect of the invention, an optical wireless local areanetwork for interconnecting a plurality of terminals having aline-of-sight optical communication function is provided, in which abase station including a light receiving function of an angle-diversitytype and a plurality of optical transmitters having directionality isprovided and the plurality of optical transmitters can separatelyperform intensity modulation. Thereby, the above-described object isachieve.

A terminal alone may be accommodated in each space cell corresponding toeach of the plurality of optical transmitters.

A far-field pattern of a light source of the optical transmitter may besatisfactorily approximated by a generalized lambertian; a halfintensity-angle φ of the light source of each of the plurality ofoptical transmitters with respect to an angle θ of each space cell maybe given by φ=C×θ (C is constant) Where C is in a range from 0.70 to1.00.

The bass station may detect a communication request optical signaltransmitted from a terminal to be communicated with the base station,and may notify the terminal of intensity data of the optical signal ordata of an optical signal/noise ratio.

The terminal may have a function of manually adjusting a direction of anoptical transmitter-receiver at a terminal side while recognizing theintensity data of the optical signal or the data of the opticalsignal/noise ratio transmitted from the base station.

Each terminal may include a transmitter having one or a plurality ofsemiconductor lasers or light emitting diodes having the same wavelengthband as that of light sources, and a photodetector having an opticalfilter for selectively attenuating light transmitted from thetransmitter of the terminal.

A wavelength band of the light source of the transmitter of eachterminal may vary among the terminals or applications.

A wavelength band of the light sources of the transmitters of the basestation may have a spectrum component different from that of the lightsource of each terminal.

The optical wireless local area network may include means for easilyremoving the optical filter.

According to another aspect of the present invention, an opticalwireless communication system for use in an optical wireless local areanetwork for interconnecting a plurality of terminals having aline-of-sight optical communication function is provided, in whichcommunication is started by a procedure including: (a) a base stationincluding a light receiving function of an angle-diversity typedetecting a communication request optical signal transmitted from eachterminal; (b) the base station comparing among a signal from eachphotodetector receiving the communication request optical signal, andselecting a photodetector having a highest optical signal intensity or ahighest optical signal/noise intensity ratio, or calculating a highestoptical signal/noise intensity ratio based on signals of a plurality ofphotodetectors, and recognizing space cells located in each terminals;(c) the terminal being notified of intensity data of the optical signalor data of an optical signal/noise ratio of the communication requestsignal from an optical transmitter forming an optical space cellcorresponding to each terminal; (d) a direction of the opticaltransmitter-receiver of the terminal being manually adjusted whilerecognizing the intensity data of the optical signal or the data of theoptical signal/noise ratio; and (e) a signal providing communicationpermission being transmitted from the base station to the terminal whenthe intensity data of the optical signal or the data of the opticalsignal/noise ratio of the communication request optical signal reaches avalue allowing communication. Thereby, the above-described object isachieved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram showing a local area network according tothe present invention.

FIG. 2 is a top view showing a configuration of space cells produced bya multi-beam transmitter according to the present invention.

FIG. 3 is a diagram showing spectrum characteristics of atransmitter-receiver unit of a terminal according to Example 1 of thepresent invention.

FIG. 4 is a diagram showing spectrum characteristics of beams of amulti-beam transmitter according to Example 1 of the present invention.

FIG. 5 is a diagram showing spectrum characteristics of atransmitter-receiver unit of a terminal according to Example 2 of thepresent invention.

FIG. 6A is an outside diagram of a card-type terminaltransmitter-receiver unit according to the present invention.

FIG. 6B is an outside diagram of a port connection-type terminaltransmitter-receiver unit according to the present invention.

FIG. 7 is a diagram showing a variety of forms of conventional opticalwireless communication.

FIG. 8 is a diagram used for explaining an angle-diversity receiver.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention is based on IM/DD. In examples of the presentinvention described below, communication protocols ormodulation/demodulation systems are not described in detail. A detaileddescription is given of the operation of an entire LAN system. Theobject of the present invention or the present invention itself iseffective in any protocol or modulation/demodulation system. In otherwords, effects of the present invention can be obtained withoutdepending on any protocol or modulation/demodulation system.

EXAMPLE 1

FIG. 1 shows an entire LAN system according to Example 1 of the presentinvention. The LAN system of Example 1 includes an optical wireless hub100 provided on a ceiling, a portable terminal 110, and a computer 111such as a desktop computer, and a printer 112. In Example 1, a card-typetransmitter-receiver unit 114 is attached to the portable terminal 110.A card-type transmitter-receiver unit 116 is attached to the printer112. A port connection-type transmitter-receiver 115 is attached to thecomputer 111. The card-type transmitter-receivers 114 and 116 and theport connection-type transmitter-receiver 115 each have an axis whosedirection can be freely changed. Structures of the transmitter-receivers114 through 116 will be described later with reference to FIGS. 6A and6B. The optical wireless hub 100 includes an imaging receiver 101 as areceiver. In FIG. 1, the transmitter-receivers 114 through 116 transmitbeams toward the imaging receiver 101. The beams are respectivelyindicated by reference numerals 120, 121, and 122.

FIG. 2 shows space cells formed by the respective beams of a multi-beamtransmitter 102 which is a transmitter of the optical wireless hub 100,the space cells being viewed 15, from directly above. The size of eachcell is determined by the directional angle of each beam of themulti-beam transmitter 102 and the height of the ceiling. In FIG. 2, thespace cells corresponding to the respective terminals 110, 111, and 112are indicated by reference numerals 210, 211, and 312. The diameter ofeach of the space cells 210, 211, and 212 is about 1 m. In FIG. 1, thebeams forming the space cells 210, 211, and 212 are indicated byreference numerals 220, 221, and 222, respectively.

Hereinafter, the optical wireless hub 100 and the space divisionmultiplexing will be described. The imaging receiver 101 which is thereceiver of the optical wireless hub 100 includes at least; an imaginglens including a plurality of lenses combined with each other asdescribed with respect to the conventional techniques; an array providedon the focal plane of the imaging lens, in which a silicon pin PD isintegrated with a monolithic, a low noise preamplifier array connectedto each cell in the array; and a multiplexer for conducting signalprocessing such as calculating a signal/noise ratio (SNR) for anindividual signal of each cell and comparing SNRs among each cell, andfor determining a cell which will be used for signal reception from acertain terminal. Further, the multi-beam transmitter 102 requires adriver dedicated to the light source of each beam so that individualsignals can be simultaneously transmitted to all of the space cells. Theoptical wireless hub 100 requires a driver circuit interconnected withboth the imaging receiver 101 and the multi-beam transmitter 102. Thedriver circuit requires a multiplexer for providing instructions such asestablishment or mediation, link management, and timing control ofcommunication among a plurality of terminals while taking into accounttemporary storage of data, instruction requests, and the like.

The spatial (angle) resolution of the imaging receiver 101 is preferablyhigher than spatial resolution determined by the sizes of the spacecells 210 through 212, i.e., the beams 220 through 222 of the multi-beamtransmitter 102. As described with respect to the conventionaltechniques, a certain cell in the PD array, which is determined by anangle which the imaging receiver 101 and incident signal light attain,and the above-described space cell formed by a beam emitted from themulti-beam transmitter 102 in a direction substantially equal to thedirection of the incident signal light must have a one-to-onecorrespondence in advance. The above-described correspondence should bedetermined as an inherent property of the optical wireless hub 100 inadvance.

In Example 1, the size (a thick solid line) of each space cell isdetermined based on the beam directional angle of the multi-beamtransmitter 102, the height of the ceiling, the position of the spacecell, and the minimum reception sensitivity of the terminal receiver.The imaging receiver 101 of the optical wireless hub 100 typically has ahigher spatial resolution. When the size of a space cell is sufficientto accommodate only one terminal, an existing IrDA terminal can beincorporated into a local area network which guarantees a high level ofthroughput at low cost and low power consumption without requiringanother electric multiplexing technique. In this case, the effects ofthe present invention can be maximally obtained. The size of anoverlapped region is appropriately determined based on the calculated ormeasured bit error rate, even though depending on the size of a deadzone tolerable for the network. As for the spatial positions of themultiple beams, the multiple beams may be a plurality of concentriccircles, or alternatively, the multiple beams may be provided directlyunder the transmitter 102 (a thin solid line).

A preferred example of a light source having a generalized lambertianfar-field pattern used as a light source of the multi-beam transmitterwill be described below. More preferably, an angle of a space cell isgiven by θ=arctan(R/D), where R is the radius of a space cellaccommodating only one terminal, and D is the distance from themulti-beam transmitter to a point at which the radiant intensity of themulti-beam transmitter reaches a peak in a space cell. D is changeddepending on a maximum possible range for communication which isdesignated by a user utilizing the present invention. The radius R isevaluated on a plane which includes a terminal and is normal to a lineconnected between the multi-beam transmitter and the point at which theradiant intensity of the multi-beam transmitter reaches a peak in aspace cell. The radius R is appropriately selected in a range which isregarded as an optimal range by a user utilizing the present invention.

In this case, it is difficult to optimize the cell positions and thesettings of the light output of the transmitter in order to obtain aslarge a coverage area as possible while taking into account interferencebetween the overlapping adjacent space cells. Theoretical andexperimental studies which the inventors of the present invention haveconducted have demonstrated that a light output required for amulti-beam transmitter is minimized in a communication distance which istypically used in a network environment of a typical small office spaceor a home space under a more general condition of φ given by φ=C×θ (C isconstant), where φ is the half-angle of a light source of a multi-beamtransmitter forming each space cell and C is selected in the range from0.70 to 1.00.

Specifically, communication was simultaneously conducted among aplurality of channels where the transmission distance D=100 to 500 cmand correspondingly the cell radius R=20 to 100 cm. Meanwhile, the biterror rates of a down link with respect to various half-angles φ wereevaluated and studied. As a result, the constant C that achieved a largecoverage area while minimizing power consumption was in the range fromabout 0.80 to about 0.90. It was found that a preferable half-angle ofthe light source of the multi-beam transmitter was obtained when C wasin such a range.

Note that the above-described result was obtained when the radius R isdefined in the plane that includes a terminal and is normal to a lineconnected between a multi-beam transmitter and the point at which theradiant intensity of the multi-beam transmitter reaches a peak in aspace cell. When the multi-beam transmitter is provided on a ceiling asin Example 1, a space cell on the peripheral portion of the plane onwhich the terminal exists is enlarged by an angle from the multi-beamtransmitter. In this case, to obtain an optimal state, a relativelysmall φ is assigned to the peripheral portion, while a relatively largeφ is assigned to space cells directly under the multi-beam transmitter.For communication distances in a network environment of a typical smalloffice space or a home space including all of the above-described cases,if C is designated in the range from 0.70 to 1.00, a down link system issimply constructed by designating the geometry of radiant intensity peakpositions of space cells. Such a simple design is effective in practicaluse.

Next, a spectrum characteristic of a transmitter-receiver of a terminalwill be described. The light sources of the beams 120 through 122 fromtransmitters of the terminals 110 through 112 shown in FIG. 1 areFabry-Perot laser diodes (LDs) of AlGaAs having a wavelength band from780 to 850 nm. The receivers of the terminals 110 through 112 aresilicon pin photodiodes (PDs).

Band-cut filters are provided around the PDs to prevent light emittedfrom the LDs from being diffracted and returned to the PDs in the sameterminals (see FIG. 5). The band-out filters have reflectance which isselectively high with respect to the wavelength of the light emittedfrom the LDs.

FIG. 3 shows light intensity spectra of the transmitters and reflectionspectra of the band-cut filter included in the receivers, of thetransmitter-receivers 114 through 116 of the respective terminals 110through 112. Specifically, the wavelength of the LD in the transmitterof the transmitter-receiver unit 114 included in the portable terminal110 is 780 nm. The center wavelength of the band-cut filter in thereceiver thereof is also set to 780 nm. The bandwidth of light cut isabout 10 nm. The band having a width of 10 nm centered at 780 nm isreferred to as the wavelength band used in the portable terminal 110.Similarly, the wavelength used in the transmitter-receiver unit 115 ofthe computer 111 is set to 800 nm, and the wavelength used in thetransmitter-receiver unit 116 of the printer 112 is set to 820 nm.

The band-cut filter can be made of a planar dielectric multi-layer film.The center wavelength, bandwidth, reflectance, and the like of theband-cut filter can be set to desired values by appropriatelydesignating materials, the number of the materials, the thickness ofeach layer, repeating patterns, and the like. A problem with the planardielectric multi-layer film is that since the optical path length ischanged with the incident angle of light, the center wavelength as wellas the incident angle of the light is shifted. However, in the presentinvention it is assumed that the transmission and recession of lighttogether form line-of-sight communication having as high a level ofdirectionality as used in communication among portable terminals.Therefore, the above-described influence of an angle shift can besufficiently reduced, and the planar dielectric multi-layer film issufficient for practical use. Needless to say, in FIG. 3, the wider thebandwidth of light cut by the band-cut filter, the less the influence ofnoise due to sun light, fluorescent lamps, incandescent lamps, or thelike.

As described above, in each terminal, a receiver includes a filter forcutting signal light emitted from its own transmitter. Each terminalperforms communication via the optical wireless hub 100 employing atransmission light source including a spectrum component of a wavelengthdifferent from that used in the terminals 110 through 112. This allowsfull-duplex communication. The space multiplexing allows each terminalto achieve multiple access in the LAN. Further, if the wavelengths usedby the terminals are different from each other, one-to-one full-duplexcommunication between the terminals as well as a simultaneous link amonga plurality of terminals can be achieved. Note that a wavelength used ineach terminal and a relationship between the wavelength and the spectraof the beams 220 through 222 of the multi-beam transmitter 102 will bedescribed in detail after describing the operation of the entire LANsystem of Example 1.

The operation of the entire LAN system in accordance with instructionswill be described. The instructions are: data is transferred from theportable terminal 110 to the computer 111, the data is added to a sharedfile; and the result is output via the printer 112. The communicationbetween the terminals 110 through 112 and the optical wireless hub 100is conducted via the directional beams shown in FIG. 1 or 2.Hereinafter, such communication is simply described as “transmissionfrom A to B” or the like if does not need to be particularly specified.

An optical axis adjuster on which the transmitter-receiver unit 114 ofthe portable terminal 110 is visually and manually adjusted with respectto the optical wireless hub 100. Light sources have a directional halfangle of about ±15° which allows easy axis alignment. In addition, thelight sources achieve eye safety in conformity with the class I of theinternational standard ISC60825-1. To this end, the diameter of emittedlight from the light sources is enlarged to 4.5 mm using lenses anddiffuse plates. The tolerable maximum outputs of the light sources aredesignated to 58 mW. Such outputs have a sufficient level of power tocause a bit error rate to be 10 to the power of −8 or less in aone-to-one communication where the transmission distance is 3 m and thecommunication rate is 100 Mbps. In this case, it is assumed that thequantum efficiency of the silicon pin PD is 0.7, and the radius ofeffective received light is 7.5 mm.

When a communication request is transmitted via the beam 120 from thetransmitter of the portable terminal 110 to the optical wireless hub100, the imaging receiver 101 (receiver) of the optical wireless hub 100receives the communication request and, as described above, canrecognize that the communication request is a signal transmitted fromthe space cell 210 in which the portable terminal 110 is located. Theoptical wireless hub 100 transmits a communication acknowledgementsignal via the beam 220 providing the space cell 210 in which theportable terminal 110 is located, among the beams of the multi-beamtransmitter 102, in order to give a communication acknowledgement to theportable terminal 110. In this case, when the optical axis alignment ofthe transmitter-receiver unit 114 of the portable terminal 110 isinsufficient so that the imaging receiver 101 cannot receive theabove-described communication request, i.e., the beam 120, it cannot berecognized that axis alignment is achieved in the portable terminal 110according to the fact that the portable terminal 110 has transmitted thecommunication request, but after being ready for a given period of timehas not transmitted the above-described communication acknowledgement.Therefore, a user again performs optical axis alignment visually andmanually. However, when the beam 120 transmitted from thetransmitter-receiver unit 114 of the portable terminal 110 is permittedto have a directional half angle of about ±15 and the transmissiondistance is. 3 m, the precision of the axis alignment is about ±70 cmand a complex function, such as automatic tracking, is not required.

Next, the operation of the optical wireless hub 100 when a link isestablished will be described. It is assumed that optical axis alignmentof the transmitter-receiver unit 114 is achieved where the imagingreceiver 101 can receive a signal, but the signal/noise ratio (SNR)thereof is insufficient. The optical wireless hub 100 can transmit data,which represents increases or decreases in the intensity of lightreceived (or SNR) by the imaging receiver 101, to the portable terminal110 in real time using the multi-beam transmitter 102 (such acommunication does not require a high bit rate). Therefore, a user ofthe transmitter-receiver unit 114 can adjust the optical axis of thetransmitter-receiver unit 114 to obtain the optimal direction of theoptical axis so that the intensity of the received light (or SNR) ismaximized based on the above-described data. The optical wireless hub100 transmits a signal representing completion of the above-describedlink establishment procedure to the portable terminal 110 when theoptical wireless hub 100 determines that a sufficient SNR forcommunication is achieved.

After completion of the above-described one-to-one link establishment,the portable terminal 110 requests the optical wireless hub 100 toexecute the above-described instruction. In this case, the portableterminal 110 transmits to the optical wireless hub 100 data to be addedto a file in the computer 111, a request for data addition to anapplication file in the computer 111, a request for transmission of thedata resulting from the addition, to the optical wireless hub 100, and arequest that the data resulting from the addition which has beentransmitted to the optical wireless hub 100 is transmitted and outputfrom the optical wireless hub 100 to the printer 112. Such instructionsare temporarily stored in a memory of the optical wireless hub 100 andthereafter executed sequentially. For the sake of simplicity, it isassumed that optical axis alignment and link establishment between theoptical wireless hub 100 and each of the computer 111 and the printer112 have been already conducted. If optical axis alignment of a fixedterminal has been once conducted using the above-described procedure, nosubsequent alignment is required. A plurality of the above-describedlink establishments can be conducted simultaneously.

Next, the optical wireless hub 100 searches the computer 111 using themulti-beam transmitter 102. In the search, in contrast to when theabove-described link is established, the multi-beam transmitter 102transmits communication requests to all of the cells. In this case,using a procedure similar to that used when the above-described opticalaxis alignment is conducted, communication between each terminal(computer 111 and printer 112) and the optical wireless hub 100 isconducted so that a content held in the terminal is recognized,Specifically, a terminal having an address requested by theabove-described instruction is searched. Alternatively, when an addressis not assigned in advance, a terminal having a file or data requestedis searched. After such a terminal has been found, the instruction issequentially executed. Specifically, data from the portable terminal 110is added to a corresponding application file in the computer 111. Thedata resulting from the addition is transmitted from the computer 111 tothe optical wireless hub 100. The data resulting from the addition whichhas been transmitted to the optical wireless hub 100 is transmitted andoutput from the optical wireless hub 100 to the printer 112. All of theprocesses discussed so far can be simultaneously conducted in parallelin the full-duplex communication using wavelength multiplexing alongwith space multiplexing, which are described in detail later. Therefore,even when requesting communication, if a terminal has conductedcommunication in a network, it is possible to achieve a satisfactorynetwork environment, unlike conventional optical wireless LANs in whicha given waiting time is required.

In Example 1, only three terminals constitute a LAN. In the LAN, eachterminal does not receive its own signal light, and conductscommunication using a light source having a spectrum component differentfrom a wavelength band. Therefore, a full-duplex multiple access LAN isobtained. When the number of terminals is increased, the one-to-onecommunication form between each terminal needs to be available in orderto avoid the overlapping of the wavelength used in each terminal as longas each terminal has the above-described filter.

Therefore, in Example 1, all of the terminals employ LDs havingdifferent wavelengths as transmitter light sources at the terminalsides. It is assumed that the transmitter light sources are limited toAlGaAs LDs (780 nm through 850 nm). Even when an interval between eachwavelength channel is 10 nm using a band-cut filter having a bandwidth(10 nm) similar to Example 2, 8 wavelength channels can be provided. Inthis case, the maximum number of terminals that are connected to a LANof Example 1 and can conduct one-to-one full-duplex communication witheach other is 8.

When each terminal employs an LD as a transmitter light source, a linewidth is well below 1 nm. An interval between each of theabove-described wavelength channels is determined by a narrow bandwidthof the band-cut filter. As described above, as for the filter, thetolerable range of an angle shift from the optical axis does not need tobe significantly increased. Alternatively, the filter may be combinedwith a condenser employing a parabolic plane or the filter may beprovided on a curvature so that the band cut width is reduced to 5 nm orless and the wavelength channels are spaced at an interval of 5 nm.Therefore, 15 channels can be provided between 780 to 850 nm. When anAlGaInP red color LD (630 to 680 nm) which is presently put intopractical use and an InGaAs/AlGaAs LD (980 nm) are used, 14 channels canbe provided if the wavelength channels are spaced at an interval of 10nm or 26 channels can be provided if the wavelength channels are spacedat an interval of 5 nm while an inexpensive silicon pin-PD is still usedin a receiver. Alternatively, in order to achieve a longer-range and ahyper high-speed link, a combination of an LD of InP/InGAsP orGaAs/GaInNAs having a long wavelength band from about 1.2 to about 1.6μm and a PD of Ge or InGaAs may be employed. Up to this point, theFabry-Perot semiconductor laser has been described as an example. Forany of the above-described materials, a distributed feedback laser or adistributed reflection laser may be employed. Further, when planar lightemitting lasers are provided in an array, a diameter of the emittedlight can be effectively increased.

Next, a relationship between a wavelength band used by a light source ofthe multi-beam transmitter 102 and a wavelength band used by eachterminal will be described. The number of beams from the multi-beamtransmitter 102 is equal to the number of cells dividing a space. Thedirection of each beam is fixed. An angle that the ceiling and the beamattain may be adjustable, but the angle is fixed during communication.Each beam may be emitted from a light source having a differentwavelength band. Preferably, a single wavelength band is basicallyemployed in view of the cost or the simplicity of a system. However,when a network space is occupied by cells, leaving no space, the cellsoverlap with one another. Further, in this case, light sources havingdifferent wavelength bands are employed for adjacent cells. Thisconfiguration is conventionally well known and is called wavelengthmultiplex communication. The size and overlap of each cell can beappropriately designated in view of a tolerable bit error rate forcommunication in a LAN.

In any case, the light beam sources of the multi-beam transmitter 102need to include spectrum components having a sufficient intensitydifferent from wavelength bands used in each terminal (110 through 112).FIG. 4 shows an example of a desired spectrum of a light source of themulti-beam transmitter 102 emitting a beam corresponding to each spacecell, when each terminal (110 through 112) employs the wavelength bandshown in FIG. 3. In FIG. 4, a solid line indicates the case where LDsare employed as light sources of the multi-beam transmitter 102. Dashedlines indicate the case where one or a plurality of LEDs are employed aslight sources of the multi-beam transmitter 102. All of the beams may beemitted from either of the LD and LED, or may be emitted from otherlight sources.

When the light sources of the beams 220 through 222 from the multi-beamtransmitter 102 are LDs, as in each terminal, it is easy to select awavelength band while avoiding that employed in each terminal. In downlink transmission from the optical wireless hub 100, LDs have anadvantage that high-speed modulation can be performed in the bandwidthof 1 GHz or more, as compared to LEDs having a modulation bandwidth ofseveral tens of MHz. When the light sources of the beams 220 through 222from the multi-beam transmitter 102 are LEDs in which a spectrum ofseveral tens of nmi has a full width at half maximum, the LDs have anadvantage that for a terminal having the LD as a light source and aband-cut filter for cutting a narrow band of about 10 nm or less, asmall portion of such a broad spectrum is cut off, thereby making iteasy to design a system. Further, in this case, when LEDs having aplurality of peak wavelengths are provided to obtain a broad spectrum of200 nm or more for a space cell, it is easier to cause the beams 220through 222 to contain a sufficient intensity of spectrum componentdifferent from the spectrum components of the wavelength band thatemployed in each terminal (110 through 112).

As described above, in the present invention, any communication can beconducted in full-duplex, so that a procedure for communication such asrequest and acknowledgement and actual communication can be conductedamong a plurality of terminals simultaneously. Therefore, the throughputof a LAN is significantly increased as compared with conventional LANs,and a satisfactory network, environment without a waiting time can beachieved.

As is apparent from the above description, the optical wireless hub 100includes a wide variety of functions, such as searching and recognitionof each terminal, transmission and reception of data and establishmentof a link between each terminal management of links simultaneouslygenerated, and temporary storage of data. With such an intelligent hub,when the optical transmitter-receiver of a terminal has a long range ora high speed, which will be achieved by conventional development, aplurality of terminals can have a multiple access capability without aload in each terminal. When the wavelengths used between each terminalis different from each other, full-duplex communication can be achievedin one-to-one directed communication among terminals connectable to theLAN.

As described above, with the present invention using a laser diode as alight source of a terminal transmitter, a hyper high-speed and largecapacity wireless LAN for portable terminals can be constructed in whichthe number of channels can be increased, a transmission rate ispotentially increased, and an improvement in communication ability inthe terminals can be directly achieved.

EXAMPLE 2

In Example 1, an LD is employed as the light source of the transmitterof a terminal. However, the LD may have an unnecessarily largecommunication capacity in a small office or home having a small numberof people. In such an environment, an inexpensive LED may be employed asthe light source of the optical transmitter of a terminal instead of therelatively expensive LD so that the cost of the entire LAN system can bereduced at the expense of a reduction in the number of channels.Hereinafter, Example 2 of the present invention will be described withreference to the accompanying drawings. In the following description,the same figures and reference numerals as used in Example 1 are usedunless otherwise indicated.

LEDs are employed as the light sources of the transmitters of theterminals 110 through 112. For this reason, which major component needsto be changed significantly from Example 1 is the band-cut filter whichis provided in the receiver of a terminal so as to prevent light emittedfrom the transmitter of the terminal from being diffracted and returnedthereto and which has a high level of reflectance selectively withrespect to a wavelength close to the wavelength of the light sourceemitting the light.

FIG. 5 shows a spectrum characteristic of a transmitter-receiver unit ofa terminal according to Example 2 of the present invention. Similar toFIG. 3, FIG. 5 shows wavelength spectra of the light sources of aterminal 1 (110), a terminal 2 (111), and a terminal 3 (112) for 3wavelength channels, and reflectance spectra of the band-cut filtersincluded in the receivers of the terminals 1, 2, and 3. The bandwidth tobe cut off is enlarged by changing the structure of a dielectricmulti-layer film in accordance with the light source of each terminal.Specifically, LEDs included in the terminals 1, 2, and 3 have wavelengthbands centered at around 800 nm, 870 nm, and 950 nm, respectively, andeach have a full width of about 40 nm at half maximum. The bandwidth ofthe filters for cutting the respective spectra is about 50 nm.Therefore, the wavelength bands of the terminals 1, 2, and 3 are spectrahaving a bandwidth of about 50 nm centered at around 800 nm, 870 nm, and950 nm, respectively.

The relationship between spectra shown in FIG. 5, i.e. designation of abandwidth to be cut off by a filter and an interval between wavelengthchannels, is not employed for multiple access using typical wavelengthmultiplexing. The relationship shown in FIG. 5 has an object to obtainfull-duplex communication by reducing a signal emitted from thetransmitter of a terminal and returned thereto to a level much less thanthat of a signal received from another transmitter, i.e., a multi-beamtransmitter. It should be noted that multiplexing is conducted by spacemultiplexing. As described in Example 1, the number of wavelengthchannels means the maximum number of terminals which are simultaneouslyconnected to a LAN according to Example 1 and which can performone-to-one full-duplex communication between each terminal.

The number of wavelength channels should be determined based on thedesign of the internal structure of a transmitter-receiver of a terminalwhose directional is angle and internal reflection are taken intoaccount, the design and fabrication technique of a band-cut filter, anda communication distance and a communication rate required in SNRevaluation in which a signal emitted from a terminal and returnedthereto is regarded as a noise source for the terminal. In anexperimental example where substantially the same communication form asused in a conventional IrDA terminal was employed, and a peeking circuitwhich enables high-speed modulation was applied to an LED and a PD, therelationship shown in FIG. 5 needed to be substantially satisfied inorder to achieve full-duplex communication at a communication distanceof 1 m, at a communication rate of 100 Mbps, and at a one-to-one link.More preferably, the cut width of a band-cut filter is increased. In theLAN system of the present invention, a band to be modulated ispreferably shifted in order not to affect communication between devices(e.g., a remote control for a TV) using an infrared ray which arelocated in the area but are not related to the LAN.

Further. Example 2 shows the case where the light source of atransmitter is made of AlGaAs and the detector of a receiver is asilicon pin-PD. Similar to Example 1, a system can be extended using avariety of materials.

Next, a light source of a multi-beam transmitter will be described.Similar to Example 1, preferably, each of the light sources providingthe beams 220 through 222 and the like corresponding to the respectivespace cells basically includes one or a plurality of LEDs having asingle wavelength band in view of the cost or the simplicity of thesystem. In this case, any one of the beams 220 through 222 and the likeof the multi-beam transmitter 102 needs to include a spectrum componenthaving a wavelength band different from that used in each terminal and asufficient intensity. In Example 2, however, since the transmitters ofthe terminals employ LEDs as light sources, a wavelength band issubstantially occupied by the transmitters of the terminals. In contrastwith Example 1, the number of terminals used is advantageously increasedwhen the light sources of the multi-beam transmitter 102 are LDs havinga signal wavelength band. Similar to FIG. 4 illustrating Example 1, theabove-described requirements can be satisfied when a very broad spectrumis achieved by combining LEDs having different peak wavelengths for eachspace cell. These cases are easily understood from FIGS. 3, 4, and 5. Noadditional figure is provided for explaining these cases.

As described above, in the present invention, any communication can beconducted in full-duplex, so that a procedure for communication, such asrequest and acknowledgement, and actual communication can be conductedamong a plurality of terminals simultaneously. Therefore, the throughputof a LAN is significantly increased as compared with conventional LANS,and a satisfactory network environment without a waiting time can beachieved.

As is apparent from the above description, the optical wireless hub 100includes a wide variety of functions, such as searching and recognitionof each terminal, transmission and reception of data and establishmentof a link between each terminal, management of links simultaneouslygenerated, and temporary storage of data. With such an intelligent hub,when the optical transmitter-receiver of a terminal has a long range orhigh speed, which will be achieved by conventional development, aplurality of terminals can have a multiple access capability without aload in each terminal. When a wavelength used between each terminal isdifferent from each other, full-duplex communication can be achieved inone-to-one directed communication among terminals connectable to theLAN.

As described above, with the present invention using a laser diode as alight source of a terminal transmitter, a hyper high-speed and largecapacity wireless LAN for portable terminals can be constructed in whichan improvement in communication ability in the terminals can be directlyachieved. Such a LAN is relatively inexpensive and well suited for smalloffices or homes.

EXAMPLE 3

In addition to the situations described in Example 1 or 2, the casewhere terminals employing light sources (LDs or LEDs) transmitting lightof the same wavelength band are located in an area will be described asExample 3.

Such a situation may emerge when a similar terminal is newly introducedto a LAN which has been previously constructed. In the LAN of thepresent invention, no problem arise. This is because bi-directionalcommunication is conducted via an optical wireless hub, where lightsources of a multi-beam transmitter have a wavelength band which can bereceived by all of the terminals, and recognition of each terminal isconducted by space division by an imaging receiver. However, terminalsemploying LDs or LEDs having the same wavelength band still cannotperform one-to-one directed communication with each other. Therefore,problems are avoided by using a removable band-cut filter included inthe transmitter of a terminal.

FIG. 6A shows an example of a card-type transmitter-receiver unit 400.FIG. 6B shows an example of a port connection-type transmitter-receiverunit 400. As shown in either of FIG. 6A or 6B, the transmitter-receiverunit 400 includes a transmitter 401 and a receiver 402, and a band-cutfilter 403 is provided outside the transmitter-receiver unit 400.

As to the size of transmitter-receiver unit 400, a plate shape of about2 cm×2 cm has been realized even in a current transmitter-receiverincluded inside a terminal. In FIG. 6A, the previously provided band-cutfilter 403 is removed from the receiver 402 adjacent the transmitter401. Alternatively, the band-cut filter 403 may not be removed, but theangle of the band-cut filter 403 may be changed so that a detector isnot shielded. The filter may be a combination of a plurality of plates,or may be in the shape of a hemisphere which surrounds a detector, asshown in FIG. 6B.

Further, as shown in FIG. 6B, preferably, the hemispherical filter maynot be removed, but may be pivoted around an axis so that the filter canbe removed from the PD. The reasons why a terminal can receive externalsignal light having the same wavelength band as that used in theterminal and communication is limited to half-duplex communication hasbeen described with respect to the conventional techniques. A one-to-onecommunication form in the state where the band-out filter 403 is removedis a current communication form between IrDA terminals. Therefore, whenan optical transmitter-receiver included in a terminal having theabove-described removable filter and a card or port adapter isincorporated into a conventional IrDA terminal, such a terminal can bevery easily connected to a wireless LAN for the hyper high-speed andlarge capacity line-of-sight optical communication terminal of thepresent invention.

When the optical transmitters of all terminals have LEDs having awavelength band of around 850 nm (the spectrum has intensity in thewavelength range from about 800 to 1000 nm) as light sources and thereceivers of all of the terminals have silicon pin-PDs, similar to acurrent IrDA terminal, the following configuration is provided as a mostpreferred example where the plurality of terminals are simultaneouslyconnected to the LAN of the present invention. Specifically, LDs allhaving a wavelength band of around 780 nm are employed as light sourcesof the multi-beam transmitter 102. Such LDs are the most inexpensive LDsthat are widely used as light sources for reading or writing data fromor to existing recording media such as CDs, CD-ROMs, MOs, and MDs. Anoptical filter which is included in the receiver of a terminal and whichselectively attenuates a signal emitted from the terminal is notnecessarily the band-cut filter having a relatively narrow band asdescribed in Example 1, or 2. An optical filter having a so-called shortpass filter characteristic which has a transmittance of almost 100% withrespect to a wavelength of 780 nm and a reflectance of almost 100% withrespect to a longer wavelength band at least including the range from790 to 1000 nm is sufficient.

As is apparent from the above description of Examples 1 and 3, whenvarious types of transmitter-receivers of the terminals are combined,i.e., a plurality of terminals having light sources such as LDs or LEDsand filters of various bandwidths are provided in the same LAN, LANconnection and one-to-one directed communication can be achieved.Therefore, a very flexible LAN which allows directed/line-of-sightcommunication technology to be progressed can be achieved.

INDUSTRIAL APPLICABILITY

As described above, according to the space-division multiplex local areanetwork of the present invention, the following advantages are obtained:

(1) the longer-distance directed/line-of-sight communication for use ina portable terminal allows a network environment in which simultaneousmultiple access and full-duplex communication are conducted among theabove-described terminals, and full-duplex communication even inone-to-one directed communication between each terminal connectable tothe network is possible;

(2) the longer-range directed/line-of-sight communication for use in aportable terminal allows a network environment in which simultaneousmultiple access and full-duplex communication are conducted among the isabove-described terminals;

(3) the longer-range directed/line-of-sight communication for use in aportable terminal allows a network environment in which simultaneousmultiple access and full-duplex communication are conducted among theabove-described terminals, so that the problem with wavelength band indirected communication between each terminal can be avoided;

(4) the optical transmitter-receiver allows a terminal having aconventional optical communication function to be connected to theabove-described hyper high-speed and large capacity wireless LAN forportable terminals; and

(5) terminals having various types of transmitter-receiver exist in thesame LAN, whereby a very flexible LAN which allowsdirected/line-of-sight communication technology to be progressed, i.e.high-speed and a long distance, can be achieved.

1. A base station for use in a space-division multiplex optical wirelesslocal area network for simultaneously interconnecting more than one of aplurality of terminals associated with the wireless local area networkto at least one other terminal associated with the wireless local areanetwork, the base station comprising: an angle diversity receiver; and amulti-beam transmitter for outputting a plurality of beams carryingoutput information from said angle diversity receiver, wherein themulti-beam transmitter includes a plurality of optical transmitters soas to form a plurality of different space cells each having apredetermined size respectively associated with corresponding ones ofsaid beams, and each of the plurality of optical transmitters includesat least one LD or at least one LED as a light source; wherein saidangle-diversity receiver includes a plurality of receiving elementsseparately associated with each respective one of said different spacecells, and at least one of the plurality of receiving elements ispositioned to receive input from each of said more than one of aplurality of terminals associated with the wireless local area networkand wherein, in the multi-beam transmitter, directional half valueangles φ of each light source of each of the plurality of opticaltransmitters are set to specific angles different from each other, thedirectional half angles being represented by φ=C×θ where C is a constantin the range from 0.70 to 1.00, and θ is an angle of each of theplurality of space cells each having a predetermined size.
 2. A basestation according to claim 1, wherein the angle-diversity receiverincludes a lens system dedicated to reception having a spatialresolution higher than a spatial resolution of the plurality of spacecells each having a predetermined size.
 3. A base station according toclaim 2, wherein a radius of a space cell provided by each of theplurality of optical transmitters is in a range from 20 cm to 100 cm ata predetermined maximum possible distance for communication.
 4. A basestation according to claim 1, wherein a radius of a space cell providedby each of the plurality of optical transmitters is in a range from 20cm to 100 cm at a predetermined maximum possible distance forcommunication.
 5. A base station according to claim 1, wherein theangle-diversity receiver includes a lens system dedicated to receptionhaving a spatial resolution higher than a spatial resolution of theplurality of space cells each having a predetermined size.
 6. A basestation according to claim 1, wherein a radius of a space cell providedby each of the plurality of optical transmitters is in a range from 20cm to 100 cm at a predetermined maximum possible distance forcommunication.
 7. A space-division multiplex optical wireless local areanetwork for simultaneously interconnecting more than one of a pluralityof terminals associated with the wireless local area network to at leastone other terminal associated with the wireless local area network via abase station, the local area network comprising: an angle-diversityreceiver; and a multi-beam transmitter for outputting a plurality ofbeams carrying output information from said angle diversity receiver,wherein the multi-beam transmitter includes a plurality of opticaltransmitters so as to form a plurality of different space cells eachhaving a predetermined size respectively associated with correspondingones of said beams, and each of the plurality of optical transmittersincludes at least one LD or at least one LED as a light source; whereinsaid angle-diversity receiver includes a plurality of receiving elementsseparately associated with each respective one of said different spacecells, and at least one of the plurality of receiving elements ispositioned to receive input from each of said more than one of aplurality of terminals associated with the wireless local area networkand wherein, in the multi-beam transmitter, directional half valueangles φ of each light source of each of the plurality of opticaltransmitters are set to specific angles different from each other, thedirectional half angles being represented by φD=C×θ where C is aconstant in the range from 0.70 to 1.00, and θ is an angle of each ofthe plurality of space cells each having a predetermined size.
 8. Aspace-division multiplex optical wireless local area network accordingto claim 7, wherein each of the plurality of terminals includes anoptical transmitter having at least one light source, theangle-diversity receiver has an optical filter for selectivelyattenuating light transmitted from the transmitter of the terminal, andmeans for easily removing the optical filter are provided.
 9. Aspace-division multiplex optical wireless local area network accordingto claim 8, wherein each of the plurality of beams output from themulti-beam transmitter of the base station includes a spectrum componenthaving a sufficient intensity different from the spectrum components ofany one of wavelength bands used by each of the plurality of terminals.10. A space-division multiplex optical wireless local area networkaccording to claim 8, wherein each of the plurality of beams output fromthe multi-beam transmitter of the base station includes at least onewavelength band used by the plurality of terminals and a spectrumcomponent having a sufficient intensity other than the at least onewavelength band.